When editing an offer, discount card square, calendar card square or subscription box item, you'll have the option to pick an image from your Media Library, so you'll want to add your images to the Media Library first before you edit an offer.

When cloning various items, images uploaded specifically to an item via one of the Upload Image options are not cloned, whereas the Media Library Image options are cloned. For this reason, and as a general convenience, it is recommended that you add any commonly-used images to your Media Library first and then choose one of the Media Library Image options.